The statements in this section merely provide background information related to this disclosure and may not constitute prior art.
Electrodeposition coating (“electrocoat”) compositions and methods are widely used in industry today. Electrocoat baths usually comprise an aqueous dispersion or emulsion including a principal film-forming epoxy resin (“polymer” and “resin” are used interchangeably in this disclosure), having ionic stabilization in water or a mixture of water and organic cosolvent. In automotive or industrial applications for which durable electrocoat films are desired, the electrocoat compositions are formulated to be curable (thermosetting) compositions. This is usually accomplished by emulsifying with the principal film-forming resin a crosslinking agent that can react with functional groups on the principal resin under appropriate conditions, such as with the application of heat, and so cure the coating. During electrodeposition, coating material containing the ionically-charged resin having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in the electrocoat bath and then applying an electrical potential between the substrate and a pole of opposite charge, for example, a stainless steel electrode. The charged coating material migrates to and deposits on the conductive substrate. The coated substrate is then heated to cure or crosslink the coating.
One of the advantages of electrocoat compositions and processes is that the applied coating composition forms a uniform and contiguous layer over a variety of metallic substrates regardless of shape or configuration. This is especially advantageous when the coating is applied as an anticorrosive coating onto a substrate having an irregular surface, such as a motor vehicle body. The even, continuous coating layer over all portions of the metallic substrate provides maximum anticorrosion effectiveness. One desirable feature for an electrodeposited (or electrocoat) coating, then is uniform edge coverage. That is, it is important for corrosion protection that the film thickness (or filmbuild) on the edges of an article is as thick as it is on the interior portions of the article. Reuter et al., U.S. Pat. No. 6,951,602 (corresponding to WO 01/02498) describes addition of a water-soluble polyvinyl alcohol (co)polymer or mixture of polyvinyl alcohol (co)polymers to electrodeposition baths for edge protection and contamination resistance; the Reuter publication also describes other methods that have been used for edge protection against corrosion.
The cured layer of electrodeposited coating is typically overcoated with one or more layers of different, additional coatings, e.g. other primer layers and topcoat layers. Film smoothness of the electrodeposited coating is another important property because a smoother electrodeposited coating should minimize the coating thicknesses of the additional layers of coatings required to achieve a smooth finish.